Tectonic, Magmatic and Geochemical Segmentation of the Global Ocean Ridge System: A Synthesis of Observations

Abstract:

To address long-standing questions on relationships between tectonic, magmatic, and geochemical segmentation of ocean ridges, we synthesized geological, geophysical and geochemical data in well-studied axial regions at a range of spreading rates. At fast and slow spreading ridges, changes in ridge properties such as axial depth, gravity anomalies, crustal thickness, and lava composition approaching or across transform faults are often comparable to those at smaller offsets. This argues against hierarchical models where transform fault-bounded segmentation arises from deeper processes than small-scale segmentation. At both fast and slow spreading rates, geophysical observations support the concept of relatively regularly spaced “principle magmatic segments,” on average 30-50 km long and fed by melt accumulations in the shallow asthenosphere. Evidence includes thicker crust at segment centers than ends along slow spreading ridges, and seismic imaging of segmented melt accumulations in the upper mantle at fast spreading ridges (Toomey et al., 2007). In the classification of Macdonald et al. (1988), principle magmatic segments correspond with 2nd order segments on slow spreading ridges and 3rd order segments on fast spreading ridges. The principle magmatic segment concept bears similarities to the spreading cell concept of Schouten et al. (1985) but it is linked to a distinct melt plumbing system in the aesthenosphere (>6 to >12 km dbs). While principle magmatic segments are well defined by geophysical observations in a number of regions, their relationship with chemical segmentation is more complex. At fast spreading rates, changes in source composition or melting parameters can coincide with offsets of all orders, and along one of the few well-studied slow-spreading ridges the great diversity of melt compositions over short spatial scales (Gale et al., 2011) must be reconciled with geophysical evidence suggesting significant focusing of melt toward segment centers.